Knife Edge, the arete of Mount Katahdin that spans between Baxter Peak and Pamola Peak, is clear for passage on Sunday, Aug. 14, 2011. This view is taken from Baxter Peak looking toward Pamola Peak, the farthest hump. The Chimney of Knife Edge is the dip just before Pamola Peak.

“We’re going to church,” said Eben Robichaud, 22, of Sebago Lake as he leaned on his hiking stick Sunday morning and looked up at First Cathedral on Mount Katahdin.

His fellow hikers laughed, endorphins zipping through their heads from the strenuous hike. Cathedral Trail, a steep route to the summit of Maine’s tallest mountain, 5,267 feet above sea level, is dotted with three massive rock structures called Cathedrals, angular granite towers that appear to be specially crafted by some supernatural force. They rear out of the earth, waking the curiosity of each hiker who passes by.

Approximately 30,000 people swarm Mount Katahdin each year to gaze in awe at its dramatic profile, the cliffs that drop for thousands of feet and the truck-size boulders littering its alpine terrain. But what exactly lies beneath the lichen-covered rock pile that is Baxter Peak?

Burrow to the core of Katahdin and you’ll find granite — miles of granite.

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Mount Katahdin, one of the largest massifs in the Appalachian Trail, was formed as a pluton, an oval mass of molten rock that solidified deep within the Earth, roughly 40 miles by 22 miles in length and width and at least three miles thick. Erosion over hundreds of millions of years removed nearly two miles of rock, exposing the tough granite pluton beneath.

“[The pluton] was a magma chamber for one or a series of large volcanoes, and we see the remains of the things that came out of the volcanoes … Traveler [Mountain] is actually a pile of volcanic debris,” said Alice Kelley, University of Maine instructor in earth sciences.

To get to Cathedral Trail, Robichaud and a crew of 12 family and friends first hiked 3.3 miles from Roaring Brook Campground to Chimney Pond.

About 1.8 miles in from Roaring Brook Campground, a short trail to the right veers to a viewpoint that overlooks a moraine, an accumulation of glacial debris formed when the last continental ice sheet filled the Wassataquoik Valley. And 0.2 miles after that, the trail meets the Basin Ponds formed from cirque glaciers. The trail ends at Chimney Pond, a circular, pristine tarn that occupies the bottom of the South Basin. Hikers are not allowed to wade or wash in its waters.

From Chimney Pond, Cathedral Trail bursts through the tree line, which reaches about 3,600 feet and 4,250 feet in elevation, and from that point onward, it’s just tundra and block fields.

“The stuff near the top [of the pluton and, therefore, the mountain] cooled faster and is finer-grained,” said Robert Johnston, senior geologist of the Maine Geological Survey. “And the rock below cooled very slowly and had time to form bigger grains of quartz, plagioclase and feldspar.”

The granite varies from light gray, biotite granite with average grain size at the base of the mountain, to brick-red granophyre at higher elevations — a transition any hiker could easily notice.

The Cathedral Trail is actually located on a dike that is as much as 700 feet thick.

“The magma chamber had formed and was cooling,” said Kelley in an effort to describe how dikes are formed. “At the point that it was pretty solid, a new intrusion of material was pushed in, and this was the granite composition, but it was finer-grained.”

As the magma rises, it intrudes into the overlying country rock or “host” rock — in this case, the Katahdin pluton. And since dikes are vertical formations, they form at a steep or near right angle to the surface.

Joints form as the rock cools and goes from being deep within the earth, under pressure from overlying rocks, and then relaxes as it reaches the surface and the rock above is eroded away, said Kelley. The Cathedrals were formed when the vertical formations of granite were cut by prominent, closely spaced vertical joints. Through weathering, widening of cracks by freezing and thawing of water, and gravity, the granite broke along the joints into regular blocks — blocks that almost look man-made because of their geometric perfection.

Cathedral Trail, which is 1.7 miles long, leads hikers along all three Cathedrals. Blue blazes painted on exceptionally large boulders required Robichaud and his fellow hikers to calculate their moves and search for handholds. But on the rocky spine, the views of the South Basin, Great Basin and the mountains to the east were well worth the hikers’ inevitable rock rash.

When they reached Baxter Peak, it was time to make a decision about which way they would descend the mile-high mountain, some trails being more dangerous than others. Sunday’s forecast predicted thunderstorms and hail, but at 1 p.m., only a few clouds floated in the blue sky.

Robichaud’s crew split in half. Seven, including Robichaud, descended the mountain by Saddle Trail, which follows the path of the Saddle Slide, the scar of an avalanche that occurred the winter of 1998-1999 on the headwall of the Great Basin. The remaining six turned east to Knife Edge, the 1.1-mile ridge linking Baxter Peak to Katahdin’s lesser peak, Pamola, said to be named after the Penobscot Indian thunder god who protects the mountain.

“We had glaciers on that mountain, and as those glaciers eroded the rock, the headwalls rode back from two sides and the rock between gets narrower and narrower, so an arete [Knife Edge] forms,” said Kelley.

Cliffs plummet down on the north side of Knife Edge for thousands of feet, and the walls on the south are only slightly less steep, a drastic slope of jagged boulders. In places, the ridge narrows to only 2 or 3 feet. The trail, often compared to a sidewalk with sheer drops on either side, is not for the faint of heart.

Coming down Helon Taylor Trail from Pamola Peak, hikers are confronted by several notable tors, rock outcrops formed by weathering, namely the wind whipping around the spine. Their curved edges crumble to the touch, littering the mountain with granite rocks and residue known as grus.

Helon Taylor narrows to the point where stunted alpine firs are scraping hikers’ legs and they may be wondering whether they’ve somehow wandered off the trail. It’s a long, monotonous climb down. But after a day of breathtaking sights, a few scratches and grus in your hiking shoes is a small price to pay.

For a more comprehensive explanation of the geology of Baxter State Park and Mount Katahdin, check out the 2010 “A Guide to the Geology of Baxter State Park” by Douglas W. Rankin and Dabney W. Caldwell. The book is $10 and may be purchased by calling the Maine Geological Survey at 287-2801.

Johnston is scheduled to give a free presentation titled “A Volcano in Baxter State Park?” about the landscape around South Branch Pond at 4 p.m. Wednesday, Sept. 7, at South Branch Pond and while hiking up Ledges Trail in Baxter State Park. For information on the Maine Geological Survey, visit maine.gov/doc/nrimc/mgs/mgs.htm or call 287-2801. For information on Baxter State Park, visit baxterstateparkauthority.com or call 723-5140.

Editor’s note: BDN reporter Aislinn Sarnacki is friends with Eben Robichaud and was one of the crew of 12 family and friends that participated in the Cathedral Trail hike.